Localized corrosion of equipment is a serious problem in many industries and processes. In particular, corrosion failures in many oil and gas production systems, oil/gas/water transmission pipelines, petrochemical and chemical processing plants, fossil fuel and nuclear power plants are in the form of localized corrosion. Localized corrosion may result in loss of production, increase in maintenance cost, environmental pollution and potential health and safety hazards, etc. It is important that the occurrence of localized corrosion is identified and the severity determined in advance of structural failure, particularly catastrophic failure. In addition, the ability of chemicals to inhibit localized corrosion needs to be determined.
Localized corrosion is the selective removal of metal by corrosion at small areas or zones on a metal surface in contact with a corrosive environment, usually a liquid. While pitting is a type of localized corrosion, the locally corrosive pits may eventually cover substantial portions of a corroded electrically conductive article's surface. Localized corrosion may occur when small local sites are attacked at a much higher rate than the rest of the surface. Localized corrosion occurs when corrosion works with other destructive forces such as stress, fatigue, erosion and chemical attacks. Localized corrosion can cause more damage than any of these destructive forces individually.
The problems resulting from localized corrosion have been dealt with for many years with variable success. Localized corrosion is highly stochastic in nature and its occurrence is fairly unpredictable. Currently, localized corrosion is studied or monitored by measuring directly relatively large features (e.g. pits) on the surface by using standard optical microscopy with limited spatial resolution. Indirect methods are also used, such as electrochemical noise, to characterize localized (e.g. localization index) corrosion.
Erosion corrosion is the corrosion of a metal which is caused or accelerated by the relative motion of the environment and the metal surface, particularly when small particles, e.g. sand, contacts the metal surface. Erosion corrosion may also be caused by fluids, such as gases (e.g. air, natural gas, etc.) and liquids (e.g. water, oil, etc.) Erosion corrosion is characterized by surface features with a directional pattern which are a direct result of the flowing media. Erosion corrosion is most prevalent in soft alloys (e.g. copper, aluminum and lead alloys), although others may be affected. Alloys which form a surface film in a corrosive environment commonly show a limiting velocity above which corrosion rapidly accelerates. Other factors such as turbulence, cavitation, impingement or galvanic effects can add to the severity of attack. Erosion corrosion is a type of corrosion produced when easily removed scales (e.g. iron carbonate) that were initially protecting the metals in the pipe are eroded and the underlying metals are corroded. Erosion corrosion is a common cause of failure in oilfield equipment. The erosive attack is often localized at changes of pipe sections, bends or elbows where there is high velocity and/or turbulent flow.
Electrochemical noise (ECN) may be defined as the spontaneous fluctuations of current and potential generated by corrosion reactions. Various methods have been used to determine corrosion rates, including a linear polarization resistance (LPR) method. In LPR a direct current (DC) signal is applied to a corroding cell consisting of two or three electrodes and the resulting DC polarization is monitored. Provided that the applied current is small and that the potential shift is less than 20 millivolts (mV), the response is linear in most cases and the measured resistance, commonly known as the polarization resistance, may be related inversely to the rate of the uniform corrosion attack. Other techniques include the application of electrochemical impedance spectroscopy (EIS) in which a sine wave current or potential is applied, in a manner similar to the linear polarization technique, and the sine wave potential or current resulting from the applied current or potential is monitored. Alternatively, a pseudo random noise signal can be applied to a corroding cell, with the electrochemical impedance obtained by time or frequency domain transformations.
Although the above techniques are widely employed, they: (1) possess limitations in that they only provide information on uniform (general) corrosion conditions because they provide an average signal for the surface of the electrode being monitored; and (2) depending upon the environment, metallic material, and corrosion type, the assumption that the corrosion rate is inversely proportional to the measured charge transfer or polarization resistance may be invalid because the corrosion is of a localized nature.
Of general background interest are U.S. Patent Application Publication 2004/0031337 A1 which relates broadly to systems of addressing pipeline anomalies prior to failure of pipeline integrity. In particular, a pipeline inspection system integrates a serviceability acceptance criteria for pipeline anomalies, specifically wrinkles, with a method of correlating ultrasonic test data to actual anomaly characteristics. U.S. Patent Application Publication 2004/0100256 A1 concerns an inspection system for detecting flaws in oil and gas well borehole ferromagnetic tubular goods. The inspection device operates inside the tubular by first saturating a tubular wall with magnetic flux. Flaws in the wall causes flux leakage, and the magnitudes of the flux leakages are measured with Hall effect sensors disposed within the inspection device. The magnitude of flux leakage is then related to the amount of ferromagnetic material loss resulting from the flaw. Eddy currents induced in the wall are also measured and combined with the Hall effect sensor measurements to define location and geometric shape of the flaw.
It would be advantageous if new methods and systems were devised to determine and/or identify corrosion types so that efforts or techniques could be taken or employed to inhibit or prevent the identified corrosion type from continuing or occurring in the first place.